Highly accurate and fast computer-controlled pan-tilt mounts have been used in the field of tracking for decades (e.g., for missile tracking, U.S. Pat. No. 3,559,937, target tracking, weapon/gun mount). Tracking mounts provide for precise and fast computer control of pan and tilt position, speed and acceleration. These tracking mounts have been intrinsically complex and costly since they typically carry large payloads (e.g., greater than 20 pounds), and they position with great accuracy (e.g., 0.01 degrees) and speed (e.g., over 300° second).
Recent advances in the fields of image processing, computer vision and robot vision have shown that active control of sensor pan-tilt position can facilitate and simplify computer computations that support a wider range of activity than a passive sensor. Advances in low cost and powerful digital signal processors (DSP), accurate and miniature solid state cameras, sensor processing algorithms, and robotics have made sensor-based control of pan-tilt position applicable to a wide range of uses. Growth in these applications has been impeded due to lack of a suitable low cost pan-tilt mount.
Motorized pan-tilt mounts have achieved widespread use in the fields of surveillance and security (e.g., U.S. Pat. Nos. 4,673,268 and 4,937,675). Often used outdoors or under harsh conditions, these mounts are often weatherized. These pan-tilt tracking mounts typically achieve medium to large payload capacity with small motors by the use of large mechanical speed reductions. Thus, they are generally too slow for most tracking applications. Though pulse-width modulation (PWM) and constant-current motor drivers achieve better motor performance (e.g., better acceleration, higher switching rate, better dynamic torque), and advances in single chip high power microelectronics have made (PWM) constant current devices economically competitive, many prior art security motor drivers used simpler voltage drivers due to their simplicity and historically lower cost.
Precision is not typically inherent in conventional designs since their mechanical speed reductions are frequently subject to backlash (e.g., as from spur gear trains), slippage (e.g., as from belt drives), and other mechanical effects. Medium-sized payloads include advanced sensors including thermal and visible cameras with high zoom factor lenses, spotlights, lasers, antenna, and other sensors and output devices. In addition, human and very simple automated pan-tilt controls in the prior art (e.g., joystick operation, or fixed scanning and position presets) are not generally amenable to integrated computer control of mount position in response to changes in sensor input
A miniature pan-tilt tracking mount was disclosed by Kahn in U.S. Pat. Nos. 5,463,432 and 5,802,412. These patents disclose an advanced positioning device and controls suitable for real-time host computer control for applications including target acquisition and tracking for cameras and antennas, stabilization, image mosaicing, and autonomous remote surveillance. These pan-tilts maintain a large ratio of motor size to armature weight, and the primary drive mechanism is a worm drive. The mechanical system in the Kahn patents is particularly well suited to smaller payloads (e.g., less than 10 pounds). The simple mechanism provides features required for many advanced applications that include extremely high accuracy, low parts count, rugged reliability, high duty-cycle, dynamic rigidity, fine resolution and high accuracy. These devices have been manufactured and sold commercially by Directed Perception, Inc., Burlingame, Calif.
Motorized pan-tilt mounts for heavy payloads have achieved widespread use in the fields of surveillance and security. These pan-tilt mounts typically achieve medium to large payload capacity with relatively small motors by the use of large mechanical speed reductions, which makes them slow for tracking applications. For those few pan-tilts in this field that provide high payload and high speed, the best positional accuracy and dynamic rigidity obtained in the field is typically only about 0.25 degree, and high duty-cycles are not generally feasible. Precision is not typically inherent in prior art designs. Many of these devices employ drive mechanisms that have poor dynamic rigidity and which can lose fine position over time, including belt drives, cable and pulley drives, and spur gear trains with substantial gear backlash. These mechanisms typically lose performance over wide temperature ranges that can be seen in outdoor applications, and some of them cannot perform continuous rotations (a requirement for many advanced applications). Some security system pan-tilts use worm drives for their heavy payload devices, but they do not provide: precise gear meshing, axis preloading that is required for high mechanical and dynamic rigidity, metal gear lubrication bath for heavy payloads and high duty cycles, fast movement, computer controls required to support advanced applications, or high motor to armature ratios.
In military and other specialized pan-tilt fields, there are pan-tilt tracking mounts that can move heavy payloads with high speed, accuracy and dynamic rigidity. To meet these requirements, designs frequently are gimbaled (using upright support yokes) in order to mount the heavy payload closer to the confluence of the pan and tilt axes, which reduce the generated torques. These gimbal designs are larger than the compact realization described by Kahn and others, and the yoke limits the payload dimensions that can be mounted within the yoke. In addition, special attention must be paid to maintaining torsional rigidity on yoke designs, which can increase overall system weight and complexity. In order to achieve high dynamic rigidity on these larger systems, more complicated bearing and preload mechanisms are frequently employed. These systems can be more complicated and expensive than those employed in the commercial and industrial sectors.
Direct drive mechanisms have been applied to pan-tilt mounts, but compared to geared systems they use larger motors and higher current levels, large motor currents can require larger and more expensive continuous rotation slip rings, very high resolution feedback encoders are required to obtain high positioning resolution with the resulting high cost and increased complexity of control and electronics, small position movements and holding can be less reliable and steady than a geared system (e.g., a large pitch angle worm), and large direct drive motors with hollow shafts are unusual and expensive so placement of slip rings and internal wiring can be problematic.
Accordingly, what is needed is a system and method that addresses the above-identified issues. The present invention addresses such a need.